1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to a dynamic semiconductor memory device which requires periodic refreshing to retain memory cell contents.
2. Description of the Related Art
With advances in semiconductor manufacturing technology, the packing density and storage capacity of dynamic semiconductor memory devices such as DRAMs (Dynamic Random Access Memories) have been increasing in recent years. In such semiconductor memory devices, a refresh operation in active mode is performed based on an externally applied refresh command, while a refresh operation in power down mode is performed by generating a clock using an oscillator or the like internal to the device, and the addresses of the memory cells to be refreshed are automatically generated by a refresh address counter incorporated in the device.
These prior art DRAMs (or SDRAMs: Synchronous DRAMs) have been configured to refresh all the memory cells in the refresh operation, whether the device is in the active mode or in the power down mode.
However, there are some applications in which the amount of information handled at a time is large but the amount of information that must be retained continuously is small, and consequently, there are many cases in which the data only in selected memory cells in the DRAM need to retained when in the power down state.
Specifically, in battery powered portable terminals (for example, portable telephones), in many cases, if only part of the data in the power-on state can be retained, all the other information need not be retained when in the power down state. However, since the prior art dynamic semiconductor memory device is configured to refresh all the memory cells in the DRAM, it has been difficult to further reduce the power consumption in the power down mode. The need for reducing the power consumption is demanded not only for battery powered portable terminals but also for various other appliances that use dynamic semiconductor memory devices.
The prior art and its associated problem will be described later, in detail, with reference to accompanying drawings.